Primary processes for manufacturing cyclohexanone include hydrogenation of phenol, liquid-phase air oxidation of cyclohexane, hydration of cyclohexene, etc. Liquid-phase air oxidation of cyclohexane is the most widely used process in the world for manufacturing cyclohexanone and cyclohexanol due to its simple yet mature procedures and its necessity of relatively low one-time fixed investment. Currently 90% of the cyclohexanone in the world is produced by cyclohexane oxidation process.
Cyclohexane oxidation process for manufacturing cyclohexanone includes such procedures as oxidation, decomposition, saponification, waste alkali separation, alkane distillation, refining, dehydrogenation, heat recycling and tail gas reclamation. During the oxidation of cyclohexane, part of cyclohexane will be overoxidized to some neutral or acidic species which will further react with neutral alcohols to give esters. Thus, apart from the target product in the cyclohexane oxidation solution, there exist various acid byproducts and other complicated organic species with unidentified constituents. In order to separate these byproducts, crude oxidation products are generally disposed with aqueous solution of sodium hydroxide in a decomposition pot, so that the acids are neutralized and the esters are converted to sodium salts of organic acids and cyclohexanol by saponification. Following the reactions, the resulting cyclohexane oxidation solution is introduced into an alkali liquor separating system wherein the organic phase is separated from the alkali liquor. The organic phase separated out is directed to an alkane rectification procedure, and part of the alkali liquor is recycled with the rest expelled.
If the waste alkali liquor separating system of the cyclohexanone apparatus does not function well, waste alkali dispersed in the cyclohexane oxidation solution in minute form will render acyloin condensation in the reboiler of the alkane distillation tower, which will foul the reboiler and therefore deteriorate the effect of heat transfer so rapidly that the amount of evaporation is badly impaired till the reboiler can not operate properly. As a result, the whole apparatus has to stop work for the feed to be discharged and the reboiler to be cleaned. This will impact the operation cycle of the apparatus and increase the consumption of feedstock. The waste alkali liquor separated out, COD of which is up to several hundreds of thousands of mg/L, will bring about pollution to environment if it is drained off directly to the surroundings. Thus, efforts have been made to find technologies suitable for disposing the waste alkali liquor in a cyclohexane oxidation solution.
In the prior art, gravity sedimentation and coalescence separation are widely used in the waste alkali liquor separation procedure for the cyclohexanone apparatus to remove the waste alkali liquor. However, gravity sedimentation, when used to separate the waste alkali liquor from a cyclohexane oxidation solution, suffers from low efficiency of separation, long time of operation and dramatic expenditure on equipments. Moreover, only large particles (50 μm) can settle down effectively while fine particles can not. The object of complete separation of the fine particles can not be fulfilled merely by means of prolonging residence time in order to sedimentate them completely by gravity. A combination of gravity sedimentation and coalescence separation exhibits the advantage of high precision of separation with a cut diameter up to 0.1 μm, but it has the drawbacks that the coalescence elements are readily blocked when the solution is not clean, the anti-shock capability of the apparatus is low, the apparatus will lose ability of separation when there is a relatively heavy amount of waste alkali liquor, and the coalescence elements needs frequent replacement which leads to high operational cost.
The major portion of the waste alkali liquor separated from the waste alkali liquor separation system of the cyclohexanone apparatus is recycled, while the rest is expelled from the apparatus. In the prior art, the waste alkali liquor expelled from the apparatus is generally disposed by neutralization with acid and/or by incineration.
In a process disclosed by U.S. Pat. No. 4,052,441, the waste alkali liquor separated from the cyclohexane oxidation solution was neutralized by adding sulfuric acid, and an oil phase and a water phase were obtained after separation, wherein the water phase was an aqueous solution of sodium sulfate and the oil phase comprised organic acids; after removing the low-boiling-point monobasic acids and water by vacuum distillation, the oil phase was cooled and crystallized to recover adipic acid; then the mother liquor obtained was redistilled to recover monobasic acids, dibasic acids, etc., and then esterized to obtain ester products. In a process disclosed by U.S. Pat. No. 6,063,958, the waste alkali liquor was neutralized using an inorganic protonic acid, and an oil phase and a water phase were obtained after separation, wherein the water phase was an aqueous solution of an inorganic salt and the oil phase was extracted using an aqueous solution of an inorganic protonic acid to obtain adipic acid and 6-hydroxyl caproic acid at a total yield of 50-55%. Although many useful substances can be recovered from the waste alkali liquor by means of neutralization with an acid, the process suffers from complicated procedures, poor purity of the recovered organic acids and low total recovery efficiency. Furthermore, a lot of organic residues remain in water which has a COD of over one hundred of thousands of mg/L, and thus needing to be incinerated or disposed otherwise.
Incineration used to dispose the waste alkali liquor can eliminate organic species, but inorganic species (e.g. bases) are left over. Following incineration, part of the sodium carbonate is introduced into a flue along with smoke, and then recovered by electrostatic adsorption. The molten sodium carbonate at the bottom of the incinerator chamber is dissolved in water and then drained off. Notwithstanding the very low COD of the aqueous solution of sodium carbonate, the result of incineration is nothing more than a conversion from a heavy pollution to a light one.
So far, the technologies for disposing the waste alkali liquor in the cyclohexanone apparatus in the prior art have never met the demand of fine separation, neither have they tackled the problem of environmental pollution. Thus, an urgent need exists for the development of a new technology suitable for disposing the waste alkali liquor in a cyclohexane oxidation solution in a long operation cycle.